Such an interband cascade laser amplifier medium is known, for example, from U.S. Pat. No. 5,799,026 or from US 2010/0097690 A1. In this case, the optical transition used for the laser activity takes place between the hole quantum film and the electron quantum film. In this case, quantum film means that on account of the thickness of the corresponding semiconductor layers and as a result of the localization of the electrons in the conduction band of the electron quantum film and of the holes in the hole quantum film on account of the band profile with respect to adjacent layers, there is a quantization of the population levels perpendicular to the layer plane. Through suitable choice of the semiconductor materials, in particular the valence band edge of the hole quantum film is energetically above the conduction band edge of the electron quantum film. As a result, the emission wavelength in the case of an optical transition of an electron from the conduction band of the electron quantum film into the valence band of the hole quantum film becomes practically independent of the respective band gaps of the semiconductor materials involved. This allows, for example, semiconductor laser emissions in a wavelength range of between 3 μm and 5 μm, which, in the case of a dependence of the optical transition on the band gap of the semiconductor materials, has not been possible in an uninterrupted fashion heretofore in continuous wave operation at room temperature. Laser emissions in the so-called medium infrared range (MWIR) between 2.5 μm and 8 μm are of interest particularly for chemical analyses, for target seeking devices or for applications in the medical field.
Since the optical transition of the electron takes place between the conduction band of the electron quantum film and the valence band of the hole quantum film, that is to say between spatially adjacent semiconductor materials, a locally indirect band transition is involved. In the present case, this is referred to as a so-called type II semiconductor laser in this context. By contrast, if the optical transition takes place locally directly between the conduction band and the valence band of the same semiconductor material, then this is referred to as a type I semiconductor laser.
InAs, InAsSb, InGaAs or InAlAs are known as semiconductor materials for the electron quantum film from U.S. Pat. No. 5,799,026. GaSb, GaInSb, GaSbAs, GaSbAs and GaAlSb are disclosed as semiconductor materials for the adjacent hole quantum film in U.S. Pat. No. 5,799,026. The position of the conduction band edges and of the valence band edges of the electron and hole quantum films is configured for optimizing the optical transition through the choice of the III-V compound semiconductors and the thickness of the quantum films.
In contrast to a semiconductor diode laser, such as is described in U.S. Pat. No. 5,793,787, for example, a unipolar transport of a single charge carrier type, that is to say either of electrons or of holes, along the laser material takes place in the case of an interband cascade laser. For this purpose, an external voltage is applied to the laser material, such that charge carriers of one type migrate into the laser material on one side and leave the laser material again on the other side. Accordingly, the entire semiconductor material of a laser of this type has a uniform charge carrier doping. An n-type doping is provided for the transport of electrons as uniform majority charge carriers; a p-type doping for the transport of holes as uniform majority charge carriers.
In order to transport further an electron that has relaxed in the amplifier region as a result of optical transition into the valence band of the hole quantum film, and in order, in particular, to be able to use said electron for the purposes of a cascade multiply for further optical transitions, the amplifier region of an interband cascade laser is always adjoined by an electron collector region and subsequently an electron injector region. The electron collector region comprises at least one collector quantum film separated by means of an electron barrier layer. Likewise, the electron injector region comprises an injector quantum film separated by means of an electron barrier layer. In this case, the valence band edge of the collector quantum film, that is to say of the third semiconductor material, is energetically adapted for taking up an electron from the valence band of the second material. The conduction band edge of the injector quantum film, that is to say of the fourth semiconductor material, is energetically adapted for taking up an electron from the valence band of the third semiconductor material. According to U.S. Pat. No. 5,799,026 or US 2010/0097690 A1, a plurality of collector and injector quantum films and barrier layers can alternate both in the electron collector region and in the electron injector region. The barrier layer used between the amplifier region and the electron collector region prevents undesirable tunneling of the electron from the electronic level of the electron quantum films without the electron having relaxed radiatively into the energetically lower energy level in the hole quantum film.
In accordance with the prior art cited, the collector quantum films are configured with regard to their thickness and the choice of semiconductor material such that, for example, the highest quantized hole level corresponds energetically approximately to the highest quantized hole level in the valence band of the hole quantum film or is reduced by comparison therewith. In particular, the approximately linear profile of the electric field resulting from the applied external voltage within the semiconductor material should be taken into consideration in this case. The electron is thus allowed in particular to tunnel resonantly from the valence band of the hole quantum film into the valence band of a collector quantum film.
In order to have the electron available again for an optical transition in a further amplifier cascade, the electron collector region is adjoined by the electron injector region. The task thereof is to transfer the electron from the valence band of the collector quantum film into an electronic level in the conduction band of the injector quantum film, such that it can relax from there via the conduction band of an adjoining electron quantum film radiatively again into the valence band of a hole quantum film.
For this purpose, the conduction band of the adjoining injector quantum films is configured through the choice of the thickness and of the semiconductor material such that, by way of example, the lowest quantized level therein, taking the field profile into consideration, is energetically identical to or lower than the highest quantized hole level in the valence band of the last collector quantum film.
GaSb, GaInSb or GaSbAs, inter alia, are known as semiconductor materials for the collector quantum film from U.S. Pat. No. 5,799,026. The materials of the electron quantum film are used as materials for the injector quantum film. On account of the field profile resulting from the external voltage within the semiconductor material, which leads to band tilting, an electron when passing through the semiconductor material can thus be used multiply for the same optical transition at different locations. For this purpose, a plurality of amplifier media are connected in series by means of a corresponding layer construction. The electron collector region takes up the electron that has undergone transition and passes it on to the electron injector region. The latter injects the electrons into the next amplifier region, where they relax again as a result of optical transition.
The barrier layers enabling the electrons to tunnel by providing the potential barriers are constructed, in accordance with U.S. Pat. No. 5,799,026, in particular from semiconductor materials such as AlSb, AlInSb, AlSbAs or AlGaSb. These materials have a relatively large band gap. The levels—relevant to the optical transition—of the electrons and holes in the conduction band and in the valence band of the adjacent layers are energetically within the band gap of the barrier layers.
In the case of a diode laser, by contrast, the optical transition is situated within the depletion zone of a p-n junction. Charge transport predominantly takes place by means of electrons in the n-doped region, and by means of holes in the p-doped region. For laser operation, the radiative recombination of electrons and holes in the depletion zone is crucial, which are injected from opposite directions (contacts). In other words, bipolar charge transport by means of electrons and holes takes place. Electrons that have relaxed radiatively in the amplifier region are not transported further. Consequently, each injected electron can contribute maximally to the emission of one photon in the device.
For the electrical connection of the laser material of an interband cascade laser, specific connection and termination layers are furthermore known from the prior art. However, the exact construction of these layers is not the subject matter of the present invention.
It is disadvantageous that, for type II semiconductor lasers, as a result of the spatial separation of electrons and holes, only relatively small regions arise in which the residence probabilities thereof appreciably overlap. Increasing the spatial overlap in spatially indirect quantum films is of crucial importance, however, in order to maximize the probability of radiative transitions of injected electrons. For this purpose, firstly it is possible to reduce the thickness of the hole quantum film and of the electron quantum film, such that a larger spatial overlap of the residence probabilities arises on account of the greater localization of the charge carriers. However, a reduction of the thickness leads to a greater quantization of the energy levels, such that the transition wavelength changes. Secondly, that can be taken into account by correspondingly adapting the weight proportions in the compound semiconductors, since this influences the band gap between valence band and conduction band.
For a diode laser according to U.S. Pat. No. 5,793,787, in which the optical transition takes place in the depletion zone of a p-n junction as a result of recombination, and wherein there is no need to perform any band adaptation for transporting the relaxed charge carriers further, in order to increase the transition probability for a type II semiconductor laser it is proposed that the hole quantum film be embedded between two adjacent electron quantum films. Without further indications, a wide range of between 1.5 nm and 7.0 nm is indicated in this case for a possible thickness of the hole quantum film. Thicknesses of the hole quantum film of between 2 nm and 4 nm are disclosed as preferred. GaInSb having an indium proportion, relative to gallium, of between 0% and 60% and, in an unspecified manner, GaSb, GaInSbAs and GaInAlSb are mentioned as materials for the hole quantum film. For the electron quantum films, materials proposed include in an unspecified manner InAs, InAsSb having an antimony content, relative to indium, of less than 50%, InAsP having a phosphorus content, relative to arsenic, of less than 50%, InAlAs having an aluminum content, relative to indium, of less than 50%, InGaAs having a gallium content, relative to indium, of less than 50%, and in an unspecified manner InAlAsSb.
Owing to the absolutely necessary band adaptation for transporting the optically relaxed charge carriers further, the materials indicated for a diode laser together with their weight proportions cannot, however, be applied to the material structure of an interband cascade laser. Rather, selection of materials therefor requires further intensive research activity.
For this purpose, the later U.S. Pat. No. 5,799,026, for an interband cascade laser, takes up the concept from U.S. Pat. No. 5,793,787 for increasing the spatial overlap of the residence probabilities of an electron and of a hole, according to which a hole quantum film is embedded between two adjacent electron quantum films. In this case, owing to the wavy form of the residence probability of the electrons, this is referred to as a so-called type II quantum W laser. The amplifier region is also called a W quantum film. U.S. Pat. No. 5,799,026 proposes, for an interband cascade laser, making the hole quantum films as thin as possible for the highest possible efficiency of the radiative transition. Owing to the required band adaptation, in an unspecified manner GaSb, GaInSb, GaSb/InSb, GaSb/GaInSb, GaSbAs, GaSb/GaAs, GaAlSb and GaSb and AlSb are proposed as materials for the hole quantum film. In a non-specific manner, InAs, InAsSb, InAs/InSb, InGaAs, InAs/GaAs, InAlAs and InAs/AlAs are mentioned as materials for the electron quantum films.
A further specification of the semiconductor materials used for improving the efficiency of a type II interband cascade laser can be gathered from US 2010/0097690 A1, filed over ten years later. A Ga1-xInxSb compound semiconductor is disclosed therein as an ideal material for the hole quantum film. In the manner of a W quantum film, said hole quantum film is embedded between two electron quantum films composed of InAs. By way of example, an indium proportion of 35%—relative to gallium—is indicated. The thickness of the hole quantum film is specified as 2.5 nm to 5 nm.
It is thus possible to obtain a spatial overlap of the residence probabilities of electrons and holes in the type II W quantum film of more than 60%. 2.5 μm to 8 μm at high temperatures are indicated as possible wavelengths of the radiative transition. It would be desirable, for this important wavelength range in the infrared, to further increase the spatial overlap of the residence probability of electrons and holes, in order thus to improve the efficiency further.